The present invention relates to a process and device for quantifying, analyzing, interpreting, enhancing and representing in computer generated image format, medical ultrasound and other video images.
Images generated by medical ultrasound scanning devices present unique problems for image process and analysis systems. Ultrasonic scanning devices use sound transducers to introduce high frequency sonic waves into the body, either through a hand-held device pressed against the body or through a specially designed transducer inserted into a body cavity such as a rectal cavity. Elements in the body reflect the sonic waves back to the transducer according to the reflection coefficients of the elements. The measured time between emission and detection of the sonic waves is proportional to the depth of the sonic wave reflecting element within the body. A visually projectable image of the reflective elements in a plane of the body can be generated by assigning reflected wave signals a gray scale value in proportion to the reflected wave amplitude, passing the signals through a variable gain amplifier to compensate for attenuation losses as the wave reflecting elements increase in depth, and displaying the results in two dimension. The two dimensional display corresponds to a plane in the body parallel to the direction of wave travel. Bodily elements in the display can be recognized by trained observers. The display can be a moving image by generating and displaying a series of repeated images on a video monitor. This process of generating and interpreting images using ultrasonic transducers and processing means is known as sonography.
The reflective characteristics of wave reflecting elements in the body are referred to in sonography as the "echogenicity" of that area of the body. A highly reflective element would appear bright in the image and is called "hyperechoic," while an element with low reflectivity would appear dark and is called "anechoic." The mixture of hyperechoic and anechoic features in a localized area is termed the "echoic texture" of that area. A uniform set of features with similar reflective coefficients is called "isoechoic." A non-uniform set of features with a broad mix of reflective coefficients, which would appear as a speckled pattern in the image, is called "hypoechoic."
The primary cause of the speckled pattern in the image is that sonic waves do not always follow a direct path from and to the transducer, but instead may be reflected off several curved or angular reflecting surfaces causing small variations in the amplitude of the reflected wave. Since the displayed gray scale value of each "pixel" (picture element) is derived from the amplitude of the reflective wave, this variation produces speckle similar in appearance to snow in a standard television image. Although speckle is not random as is snow in a standard television image, the exact form of speckle in an ultrasound image is virtually impossible to predict because of the extraordinarily complex configuration of body tissues.
Speckle accounts for over 90% of the contents of many ultrasound images, and has been the considered a major cause of the poor quality of those images. Because speckle clouds the image and resembles snow in a television image, it is treated as noise. However, from the explanation above, it can be seen that the characteristics of speckle directly relate to the physical and echoic structure of the tissue being scanned. Thus, existing methods that suppress speckle also suppress valuable information regarding the tissue.
For example, it has been found that the several regions of cancerous tumors of the prostate gland have fairly characteristic echoic textures during the various growth stages. This phenomena is discussed somewhat in a scholarly article entitled "The Use of Transrectal Ultrasound in the Diagnosis, Guided Biopsy, Staging and Screening of Prostate Cancer," published in Volume 7, Number 4 of RadioGraphics, July, 1987. However, prior to the present invention, ultrasonic images were of a quality and resolution too poor for reliable diagnosis based upon echoic textures. Further, no procedure had been devised for the accurate quantification of echoic texture. Instead, diagnosis relied mainly on the experience of the operator.
Apart from methods for analyzing speckle, there are many existing devices and methods aimed at suppressing noise in video images. These devices and methods are primarily for use in standard television images in which noise is manifested as discreet light or dark random spots a few pixels in diameter. Most of the existing methods and devices are not specifically directed toward the unusual problems encountered in ultrasound images. In fact, these methods often suppress speckle information critical to interpreting and analyzing ultrasound images.